The wood products industry has been beset with many problems with pollution control equipment and associated ductwork, including maintenance problems, unplanned downtime due to material build up in scrubbers and wet electrostatic precipitators (WESP) and in thermal oxidizer systems. Such thermal oxidizer systems include thermal catalytic oxidizer (TCO), regenerative thermal oxidizer (RTO) and regenerative catalytic oxidizer (RCO). Deposition of particulates and inorganic salts on ceramic media in such systems, causes plugging and deterioration of the ceramic media from chemical attack and blinding and loss of effectiveness of catalysts.
In typical installations, moist wood chips, sawdust, wood fiber or other wood products are dried in a dryer using a hot air stream and the dried wood products are transported in the air stream to a cyclone separator, where the wood chips are separated from the air stream and collected for processing into saleable products, such as panel board.
The air stream exiting the cyclone separator contains a variety of pollutants, including organic and inorganic vapours and particulates, hazardous air pollutants (HAPS) and volatile organic compounds (VOCs), which must be removed before the air stream can be vented from the system.
The air stream usually is passed through a transport duct to a quench duct or chamber, wherein the temperature of the air stream is cooled by water fed by hydraulic quench nozzles which are intended to saturate the air stream with moisture. Alternatively, the air stream may be fed through an insulated duct equipped with fire protection equipment in the form of water sprinklers. The cooled air stream then is fed to a scrubber for removal of a portion of the pollutants and then to the WESP for further pollutant removal.
A problem exists with respect to the duct leading to the quench section and recirculation duct to the energy system or dryer and that is the build-up of materials in the duct as a result of condensation of condensable materials and dropout of fibre and particulates as the temperature of the gas stream falls, causing maintenance problems and a fire hazard.
The present invention seeks to solve the connecting, recirculation and transport duct problems by modification to the pollution control equipment and procedures used in the wood products industry. The invention is illustrated, as disclosed below, by reference to pollution control of sawdust and wood chip dryer emissions, but the principles are applicable to pollution control to other wood products and biomass dryers and other sources of gas-borne pollution which lead to contaminant build up.